This invention relates generally to integrated circuits, and in particular to an on-chip trim link sensing and latching circuit for fuse links.
Integrated circuits on which trimming of critical circuit parameters is required often employ fuse links to achieve the trimming. A fuse link is a low impedance material that can be blown like a fuse such as by driving a large current through the fuse link, or left in tact, that is not blown. Known as programming, the blowing or leaving in tact of fuse links typically occurs when the integrated circuit is tested subsequent to manufacture. During the testing, the fuse links are programmed to trim such parameters as current, voltage, or frequency to overcome processing variations and to generate a precise parameter as a reference.
Metal fuse links, which in an unblown state are a low impedance, may be xe2x80x9cblownxe2x80x9d or xe2x80x9copenedxe2x80x9d by passing a current large enough to destroy the fuse link, resulting in a high impedance. Thus, a fuse link that is not blown may represent a first logic state and a fuse link that is blown may represent a second, opposite, logic state. Blowing a fuse link causes the fuse link impedance to change from a relatively low impedance typically in the range of ten ohms to five hundred ohms, to a relatively high impedance ideally in the range of megaohms. When fuse links are blown, the resulting impedance can vary over a large range with some blown fuse links exhibiting an impedance as low as three to five kilo-ohms.
Determining whether a fuse link is blown or not has typically been achieved using an analog sense amplifier. Analog sense amplifiers, however, have the shortcoming of a threshold level of sensitivity within the range of impedances that a blown fuse link may have in that they can not sense an impedance less than a threshold, or minimum impedance. As a result, using an analog sense amplifier to determine whether a fuse link is blown or not blown could result in an erroneous indication that a fuse link was not blown, when in fact it was blown. Furthermore, sense amplifiers require biasing to operate properly.
It is desirable to have an improved technique for sensing whether fuse links are blown or not blown that does not have the shortcoming of the threshold level of sensitivity within the range of impedances that a blown fuse link may have.
In accordance with the invention, an integrated circuit includes a pulse generator for generating a pulse of a predetermined duration. A first switch, controlled by the pulse, drives current into a fuse link when the pulse takes on a first logic level. The first switch prevents flow of current into the fuse link when the pulse takes on a second logic level. A latch is coupled to the fuse link to sense a logic level developed during the pulse. The latch may be cleared by the leading edge of the pulse. The logic level developed at the fuse link due to the driven current is latched into the latch by the trailing edge of the pulse and is indicative of whether the fuse link is blown or not blown.